In a plurality of technical applications it is desirable these days to operate an electronic circuit at an operating voltage which may vary within a wide range. In automobile applications, the operating voltage may vary, for example, between 3 volts and 34 volts. In addition, it is necessary to provide protection against polarity inversion of the operating voltage. For reasons of cost and reliability, the circuit for voltage stabilization and the reverse-connect protection are to be integrated on the integrated circuit to be supplied. To keep costs low and yield high, the additional technological expenditure due to monolithic integration of the circuits for voltage stabilization and for reverse-connect protection is to be as small as possible. It is admissible, for example, to use one to two additional uncritical masks in comparison with a conventional process. In addition, the circuit for voltage stabilization and reverse-connect protection is to be robust against technological variations.
Since the circuitries for voltage stabilization and for protection against polarity inversion are typically considered a unit and are also partly realized by one electronic component, it is necessary to consider the requirements placed upon the overall system consisting of voltage stabilization and reverse-connect protection.
The regulator mostly is required to provide good suppression of external operating-voltage variations, even at high frequencies, and to react in a very robust manner to other electrical interferences. This is to be achieved without using any external blocking capacities, since they increase the number of pins of the integrated circuit and cause additional cost. In addition, it is very important for the voltage drop across the overall circuit for voltage stabilization and reverse-connect protection to be as small as possible. A voltage drop of only 0.2 volt to 0.4 volt as a maximum between the external and internal supply voltages is desirable.
Currently, several circuitries are known which can realize both reverse-connect protection and voltage regulation. Typically, a suitably driven transistor serves as a voltage-regulating element, it being possible to use both bipolar transistors and field-effect transistors. One must differentiate between circuitries wherein the reverse-connect protection is guaranteed by the regulating transistor without any further expenditure in terms of circuitry, and circuitries wherein additional measures must be taken in terms of circuitry, in addition to the regulating transistor, to guarantee reverse-connect protection. When using high-voltage pnp bipolar transistors, reverse-connect protection is ensured by the vary sequence of layers of the transistor, and no additional reverse-connect protection diode is required. However, when using high-voltage pnp bipolar transistors, suppression of high-frequency interferences on the external supply voltage is poor due to the circuit concept required here and to the resulting high parasitic capacitances. Improving the circuit properties by using a vertical pnp bipolar transistor requires additional technology steps and is therefore not desirable. Similarly, with some technologies the production of high-voltage pMOS field-effect transistors requires additional technology steps. When using high-voltage pMOS field-effect transistors, reverse-connect protection is not a matter-of-course condition, since the n trough (=bulk) of the transistor is mostly connected to the external operating-voltage terminal and forms a parasitic diode to the p substrate (=mass). With this transistor type, protection against polarity inversion is possible only to a limited extent if the bulk terminal is not directly connected to the operating-voltage pin. Here, however, there is a risk of latch-up. If the bulk terminal is connected, via a resistor, to the operating-voltage pin and the source terminal of the transistor, a relatively large current may flow through the parasitic pnp transistor in reverse operation.
If high-voltage npn bipolar transistors or high-voltage nMOS field-effect transistors, which are connected as emitter followers or source followers, are used as regulating transistors, protection against polarity inversion is not ensured by the regulating transistor itself, since a parasitic diode to the substrate exists at the collector or drain. For reverse-connect protection, additional high-voltage pnp bipolar transistors or high-voltage diodes must be used. This increases the entire voltage drop across the regulating transistor and reverse-connect protection circuit with conventional circuitries to about 0.8 volt to 1 volt. In addition, npn bipolar transistors can be produced with additional technology only.
Furthermore, the drive circuit for the transistors varies depending on the type of transistor used. With high-voltage pnp bipolar transistors, driving may be effected via a current source connected to the reference potential. If a high-voltage pMOS field-effect transistor is used as a regulating transistor, same requires a voltage drive related to the external operating voltage. A high-voltage npn bipolar transistor is driven by a base current, the base potential being generally positive towards the internal operating voltage. When using a high-voltage nMOS field-effect transistor of the enhancement-mode type, the gate potential is, in normal operation, positive towards the regulated supply voltage. It is possible to achieve such a positive potential towards the internal supply voltage by means of a charge pump, this resulting, however, in a very slow control behavior since the gate reload current can only be very small due to the load pump.
With reference to FIGS. 4 and 5, some embodiments of monolithically integratable current supply circuits with reverse-connect protection and voltage regulation in accordance with the prior art will be explained in more detail.
FIG. 4 shows a current supply circuit designated by 10 in its entirety and which creates an internal voltage supply VDDint of 2.5 volts due to an external supply voltage VDDext which may vary between 3 and 34 volts. A lateral high-voltage pnp bipolar transistor 12 is connected, as a regulating transistor, between the external supply voltage VDDext and the internal supply voltage VDDint. This bipolar transistor 12 is used as a regulating transistor and ensures reverse-connect protection. The drive signal for bipolar transistor 12 is generated by a voltage regulation circuit 14 consisting of a “band gap” reference voltage source 16 and a transconductance amplifier 18 coupled thereto. A high-voltage n-channel MOSFET 20 is connected between the output of the transconductance amplifier 18 and the base terminal of bipolar transistor 12, as a high-voltage cascode so as to decouple the output of the transconductance amplifier from the high voltage at the base of the pnp transistor. The gate terminal of this field-effect transistor 20 is connected to the internal voltage supply VDDint. In addition, in the present circuit, there is an inevitable parasitic capacitance Cpar between the base terminal of the bipolar transistor 12 and the reference potential GND. In addition, the circuitry includes a sensor circuit 22 supplied by the internal supply voltage VDDint. All circuit components use the same reference potential GND.
In the circuitry of claim 4, the lateral high-voltage pnp bipolar transistor 12 acts as a regulating transistor and as reverse-connect protection at the same time. However, such a circuitry is sensitive towards high-frequency glitches on the external supply voltage VDDext. This is due, in particular, to the parasitic capacitance Cpar which fixes the base potential with respect to alternating voltage. Thus, high-frequency interferences on the external supply voltage VDDext strongly impact the voltage across the base-emitter path of the bipolar transistor 12, which results in poor suppression of high-frequency interferences on the external supply voltage VDDext. Slow variations of the external supply voltage VDDext, however, may be regulated via the voltage regulation circuit 14, via the field-effect transistor 20 used for decoupling the bipolar transistor from the regulating circuit, and via regulating transistor 12, so that the internal supply voltage is maintained constant. It is possible to replace the lateral high-voltage pnp bipolar transistor 12 by a vertical pnp bipolar transistor. Even though this reduces the parasitic capacitance and thus improves the circuit's behavior toward high-frequency interferences on the external supply voltage, it requires additional technology steps, which clearly increases manufacturing cost and reduces yield.
FIG. 5 shows the circuit diagram of a further embodiment of a current supply circuit with reverse-connect protection in accordance with the prior art, designated by 30 in its entirety. From an external supply voltage VDDext in the range of 3.5 volts to 34 volts, an internal supply voltage VDDint of 2.5 volts is generated. The reverse-connect protection here is achieved by a lateral high-voltage pnp bipolar transistor 32 which is clamped as a diode, i.e. its base and collector terminal are short-circuited. The emitter terminal is connected to the external supply voltage VDDext. A high-voltage npn bipolar transistor 34 is connected in series with this pnp bipolar transistor, the collector terminal of said high-voltage npn bipolar transistor 34 being connected to the collector terminal of the pnp bipolar transistor 32, and the emitter terminal of the high-voltage npn bipolar transistor 34 having applied thereto the internal supply voltage VDDint. Regulation of the internal supply voltage VDDint is performed, in turn, by a voltage regulation circuit 14 consisting of a “band gap” reference voltage source 16 and a transconductance amplifier 18. The control current available at the output of the transconductance amplifier 18 is supplied to the base of the regulating transistor 34 via a further high-voltage npn bipolar transistor 36 which is operated in the common-base circuit and acts as a high-voltage cascode. In addition, the constant-current source 38 is connected between the external supply voltage VDDext and the base of the regulating transistor 34 so as to allow an upward adjustment of the internal supply voltage via the high-voltage npn bipolar transistor 34. A sensor circuit 22, in turn, is supplied with the internal supply voltage VDDint.
In the present embodiment, a lateral high-voltage pnp bipolar transistor which is switched as a diode is thus used as reverse-connect protection. Here, a voltage drop of about 0.6 to 0.7 volt is to be expected across the emitter-collector path of the bipolar transistor 32. In addition, there is also a slight voltage drop across the collector-emitter path of the npn regulating transistor 34. Thus, the entire voltage drop across the reverse-connect protection circuit and the regulating transistor is about 0.8 to 1 volt. Thus, the external supply voltage VDDext must amount to at least 3.5 volts to be able to ensure internal supply voltage of 2.5 volts. Consequently, one may state that the circuitry shown does not meet the requirements placed upon it with regard to a small voltage drop. Thus, it is not suited for being employed in an environment with the above-mentioned specifications. What also is to be stated is that the production of the two npn bipolar transistors 34, 36 requires additional technology steps in comparison with standard CMOS technology. This, too, is unfavorable with regard to the goal of low manufacturing cost.
Further circuitries for voltage supply in accordance with the prior art may be found in the following patents: U.S. Pat. No. 5,530,394; U.S. Pat. No. 5,212,456; U.S. Pat. No. 5,596,265; U.S. Pat. No. 6,005,378; U.S. Pat. No. 6,137,276; U.S. Pat. No. 6,504,424.